1. Field of the Invention
The present invention relates to a method and an apparatus for ironing and trimming a workpiece or blank which has a cylindrical portion and a radial bottom portion formed at one axial end of the cylindrical portion. More particularly, the invention is concerned with improved method and apparatus for ironing the cylindrical portion of such workpiece, and trimming the axial end of the cylindrical portion remote from the radial bottom portion.
2. Discussion of the Prior Art
An article or product in the form of a cup or container is obtained by subjecting a deep-drawn cup-like workpiece to a trimming operation, in which the axial open end of the cylindrical portion of the workpiece is trimmed. For trimming the axial open end of the cylindrical portion, the following three methods are known in the art.
The first trimming method is illustrated in FIGS. 24 and 25. Initially, a deep-drawn cup-like workpiece 500 having an outward flange at the open end is set in a die, and the outward flange is cut at a selected radial position by a punch, as shown in FIG. 24. The radial position at which the outward flange is cut determines the final depth or height of the article to be obtained as an end product 500 as shown in FIG. 25. Then, the workpiece 500 whose outward flange has been trimmed is subjected to a kind of drawing operation for straightening the trimmed outward flange while ironing the workpiece to reduce the wall thickness of the cylindrical portion, as shown in FIG. 25, whereby the end product 502 is produced.
The second trimming method includes the steps of: preparing a non-flanged workpiece 504 by either straightening the deep-drawn flanged cup-like workpiece 500 or punching off the entire portion of the outward flange of the cup-like workpiece 500; and trimming the axial open end of the thus prepared non-flanged workpiece 504, by using a shearing punch 506 which is moved in different radial directions of the workpiece 504, so that successive circumferential parts of the open end portion of the workpiece 504 are sequentially removed by shearing cuts effected by respective radial movements of the punch 506 relative to the workpiece 504, as indicated in FIG. 26.
In the third trimming method, the non-flanged workpiece 504 is first prepared as described above with respect to the second trimming method. Then, the open end portion of the workpiece 504 is continuously cut along the circumference of the cylindrical wall, by using a shearing roller 508. As shown in FIG. 27, the roller 508 which has a shearing blade on its outer circumference is rotatable about an axis parallel to the center line of the workpiece 504, and is moved along a circle coaxial with the workpiece so that the open end of the cylindrical wall of the workpiece 504 is continuously cut off by a continuous shearing cut effected by a circular movement of the roller 508.
However, all of the three trimming methods described above suffer from a considerable amount of shear droop at the trimmed open end of the end product, which is caused by cracking of the workpiece material due to penetration of the shearing punch or roller into the material.
The shear droop means a deviation of the actual shape or configuration of the open end face of the product from the nominal shape. For example, the open end face of the product may have curved or rounded edges, instead of the nominal right-angled or chamfered edges, or the actual radius of curvature of the edges may deviate from the nominal or specified radius, for instance, may be larger than the specified radius.
In particular, the first trimming method suffers from difficulty in assuring a high degree of accuracy of the height (depth) of the end product, even if the trimming of the outward flange is effected with high dimensional precision with the desired final height of the end product taken into account. Namely, the height of the end product is also determined by the wall thickness of the workpiece which is reduced by the ironing while the outward flange is straightened by the punch. However, the wall thickness of the workpiece usually has a certain amount of error, which inevitably results in an error in the height dimension of the end product.
The second and third trimming methods have another drawback. In these methods, the trimming is effected by the radial shearing cut or cuts at a selected axial position of the workpiece, whereby the height dimension of the end product can be controlled with high precision. However, the trimming is not effected at one time in these trimming methods, that is, effected by a continuous shearing cut or successive shearing cuts along the circumference of the cylindrical wall of the workpiece. Accordingly, the open end portion of the cylindrical wall of the workpiece is subjected to different shearing forces at different circumferential positions thereof, whereby the end product obtained tends to have some deformation in the cross sectional shape. The second trimming method which requires successive radial shearing cuts to trim the open end of the workpiece also suffers from a problem that the trimmed end face of the product obtained has low flatness or straightness, namely, more or less has raised and recessed portions because the successive radial shearing cuts are not performed under the same condition.
The third trimming method has a further problem that a device for moving the shearing roll 508 along the predetermined circular path is expensive.
Although the shear droop experienced in the prior art discussed above may be reduced by minimizing the n-value (work hardening exponent) of the workpiece, the reduction of the n-value results in another problem, namely, considerable deterioration of the formability of the workpiece, for example, reduction in the ease of bending or drawing of the workpiece. Therefore, the reduction in the n-value of the workpiece is not a practical solution to this problem.